This invention relates generally to antennas and, more particularly, to high frequency antenna arrays of the type used in communication systems, such as cellular telephone systems. In cellular systems, portable telephones communicate with nearby base stations, which are themselves interconnected by land lines or other means. Each base station antenna has to have the ability to communicate with multiple portable telephones located in a geographic xe2x80x9ccellxe2x80x9d over which the base station provides coverage. Therefore, the base station antenna must have a radiation pattern extending over a full 360xc2x0 of azimuth angle. Typically, a base station antenna has three equal arrays that are angularly spaced 120xc2x0 apart, with the radiation patterns overlapping slightly to provide the required full-circle coverage.
Although the technology of such antennas is now well established, some significant difficulties have emerged concerning their placement and operation, particularly in urban and suburban areas. The antennas must be placed about fifty feet above ground and, for optimum operation, they must be visible over a direct line of sight from each telephone user. Unfortunately, conventional base station antennas do not have an attractive appearance. Also, because the antenna arrays consist of multiple horizontal elements, they provide a convenient perching place for birds, which are exposed to intense high-frequency radiation. Many communities, although wanting to maintain cellular coverage, have sought ways to hide or disguise the appearance of base station antennas. One approach is to locate the antennas in trees, or even to construct the antennas to look like trees. Whether these approaches help make the antennas less of an eyesore is still debatable. Without question, even the disguised antennas remain an attractive nuisance for birds and other small animals.
A significant design difficulty with antennas of this general type arises from the difficulty of constructing an antenna array without employing a number of metal-to-metal junctions with dissimilar metals. Over time, corrosion at such junctions may result in electrochemically induced intermodulation. In essence, a degraded metal-to-metal junction may act as a diode in the antenna structure and produce unwanted signal components that degrade antenna performance. Therefore, it is highly desirable to eliminate or minimize metal-to-metal junctions in the antenna construction. Another important issue is antenna cost. With the continuing proliferation of cellular and similar communication systems, more and more base stations antennas are needed, and constructing them at a competitive cost has become increasingly important.
Accordingly, there is a need for a base station antenna array that meets stringent engineering requirements, as well as aesthetic cost requirements. The present invention satisfies this need.
The present invention resides in a multiple-array antenna that can be mounted inside a pole. Briefly, and in general terms, the invention may be defined as a radio-frequency (RF) pole antenna with multiple arrays, the antenna comprising a ground plane structure, a plurality of antenna feed circuit boards, a plurality of arrays of antenna patches, a plurality of pairs of RF feed connectors, and a cylindrical cover for the antenna.
More specifically, the ground plane structure has a plurality (n) of structurally and electrically connected facets directed in uniformly spaced angular directions and there is a plurality (n) of antenna feed printed circuit boards. Each of the antenna feed printed circuit boards is attached to, but spaced apart from, one of the ground plane facets, and each antenna feed printed circuit board has two feed points and two symmetrical circuit paths for feeding RF signals of different polarizations. Each of the circuit paths has divergent branches leading to a plurality (m) of antenna patch drive segments. Each array of antenna patches is distributed along one of the antenna feed printed circuit boards and is mounted to provide electromagnetic coupling between each antenna patch and an associated pair of antenna feed patch drive segments, one from each circuit path in the antenna feed printed circuit board. Each antenna patch is coupled simultaneously to its associated pair of antenna feed patch drive segments, and each antenna patch includes a drive element electromagnetically coupled to its associated pair of antenna feed patch drive segments, and at least one parasitic element mounted in a spaced relationship with the drive element. Each pair of RF feed connectors provides electromagnetic coupling with respective feed points on one of the antenna feed printed circuit boards, and provides connection to RF transmitting and receiving circuitry that employ the pole antenna. The cylindrical cover encloses the entire antenna, and renders the entire assembly highly suited for mounting on a support pole of similar diameter to that of the cover.
An important aspect of the invention is that each antenna array, formed by the ground plane structure, one of the antenna feed printed circuit boards, one of the arrays of antenna patches, and one of the pairs of RF feed connectors, has metal-to-metal connection only in the pair of RF feed connectors. This minimizes intermodulation effects on antenna performance. Further reduction in intermodulation effects is obtained as a result of assembling the ground plane structure using a dimple welding process.
In the disclosed embodiment of the invention, the number (n) of antenna arrays and ground plane facets is three, and each antenna patch has two parasitic elements, including a first parasitic element mounted in a parallel spaced relationship with the drive element, and a second parasitic element mounted in a parallel spaced relationship with the first parasitic element. Specifically, the drive element in each antenna patch is a flat plate of generally octagonal shape. The first parasitic element in each antenna patch is a flat plate of irregular shape having four extending arms and diagonally slanting edges between the arms, and the second parasitic element in each antenna patch is a flat plate having an approximately square shape with diagonally cutoff corners.
In the illustrated embodiment of the invention, each array of antenna patches is driven simultaneously in two different polarization modes to provide polarization diversity gain. In particular, each array of antenna patches is driven simultaneously in linear polarization modes at +45xc2x0 and xe2x88x9245xc2x0 with respect to a vertical axis of the pole antenna.
It will be appreciated from the foregoing that the present invention represents a significant improvement over prior antennas of the same general type. In particular, the pole antenna of the present invention provides electrical performance equal to or exceeding that of competitive antennas, but is accommodated in a relatively small-diameter cylindrical cover that is mountable on a support pole of similar diameter. The pole antenna has good azimuth and elevation coverage, and low intermodulation effects, which result from the minimization of metal-to-metal joints. Other aspects and advantages of the invention will be apparent from the following more detailed description, taken in conjunction with the accompanying drawings.